1. Field of the Invention
The present invention is directed to a telemetry system and, in particular, to a telemetry system in which an RF communication signal generated by a first device (e.g., an external device) is DC balanced encoded prior to transmission to a second device (e.g., an internal device) to optimize robustness of the wireless communication link and/or maintain a substantially constant average induced voltage in the second device irrespective of the data being transmitted.
2. Description of Related Art
In a variety of scientific, industrial, and medically related applications, it may be desirable to transfer energy and power (energy per unit time) across some type of boundary. For example, one or more devices that require power (e.g., electrical, mechanical, optical, and acoustic devices) may be located within the confines of a closed system, or “body,” in which it may be difficult and/or undesirable to also include a substantial and/or long term source of power. The closed system or body may be delimited by various types of physical boundaries, and the system internal to the boundary may be living or inanimate, may perform a variety of functions, and may have a variety of operational and physical requirements and/or constraints. In some cases, such requirements and constraints may make the implementation of a substantial and/or long term “internal” power source for internally located devices problematic.
One common example of a closed system is the human body. In some medically related and scientific applications, a variety of prosthetic and other medical devices that require power may be surgically implanted within various portions of the body. Some examples of such devices include, but are not limited to, drug infusion pumps, pacemakers, defribllators, cochlear implants, sensors and stimulators.
Accordingly, in some medical implant applications, “transcutaneous energy transfer” (TET) devices are employed to transfer energy from outside the body to inside the body, to provide power to one or more implanted prostheses or devices from an external power source. One example of a conventional TET device is a transformer that includes a primary winding (or coil) external to the body and a secondary winding internal to the body. Both the primary and secondary windings generally are placed proximate to respective outer and inner layers of a patient's skin; hence, the term “transcutaneous” commonly refers to energy transfer “through the skin.” Thus, the RF communication signal generated by the external device includes both a data stream signal and an RF energy signal. When received at the implantable medical device, the RF energy induces a voltage therein. This induced voltage may be utilized to power one or more components of the implantable medical device thereby reducing the consumption of energy drawn from an internal power supply that requires surgery to replace.
Heretofore in conventional telemetry systems, a standard binary encoding scheme (i.e., a low level state for “0”s and a high level state for “1”s) and amplitude shift keying (ASK) modulation have been employed, wherein full power (maximum level) is emitted from the external device when transmitting a “1” while reduced energy (minimum level) is emitted from the external device when transmitting a “0”. Accordingly, the amount of power received by the implantable medical device fluctuates, that is, a minimum level of energy is received when transmitting a “0” bit while a maximum level of energy is received during transmission of a “1” bit. A string of successive “1”s produces a relatively high level of power that may potentially exceed the maximum threshold for proper operation of the implantable medical device. In the case in which the external device transmits a string of successive “0”s then the internal device receives a reduced energy level. If the telemetry system is a passive telemetry system whereby some of the power or energy necessary to operate at least one component in the implantable medical device is provided by the passive power source, it is possible that an insufficient amount of energy may be received by the implantable medical device if the data stream includes a relatively long duration of successive “0” bits. For instance, if the data stream is “1000000001” then during the eight successive “0” bits the implantable medical device receives a reduced energy level. It is desirable in a passive telemetry system to maintain a substantially constant average energy level induced in the implantable medical device irrespective of the data being transmitted.
Another problem associated with using a binary encoding scheme is that the received power levels in the implantable medical device associated with the high and low bits differs based on the distance separation between the antennas of the external and internal devices. As a matter of convenience the external device is typically portable relative to that of the implantable medical device. Therefore, variations in the distance separation between the coils of the external and internal devices relative to one another will cause fluctuations in the power level received by the implantable medical device for the associated bits. In general there is an inverse relationship between the coil separation distance and the power level of the bit received by the implantable medical device. That is, the smaller the distance separation between the two coils relative to one another the higher the bit power level received by the implantable medical device. As the separation distance between coils increases the received bit power level decreases. By way of example, when the distance separation between the coils of the respective external and internal devices is relatively small then a “1” bit may be received at a power level of 5 while a “0” bit is received at a power level of 3. A result of a difference of 2 is obtained between the high and low bit power levels. On the other hand, at a relatively large separation distance between the coils of the respective external and internal device a “1” bit may be received at the implantable medical device at a power level of 3 while the “0” bit may be received with a power level of 2. Under this second set of exemplary conditions, the difference in power level between the high and low bits is 1. Accordingly, the difference in high and low bit power levels varies depending on the distance separation of the external and internal coils. The farther the distance separation between the coils the smaller the difference in received power levels between the high and low bits, whereas the shorter the distance separation the greater the difference in received power levels between the high and low bits.
Variations in the difference in power level of the associated high and low bits based on the coupling distance between the coils of the external and implantable medical devices complicates recovery of the original data signal. A conventional wireless communication receiver as found in an internal device typically includes a demodulator (e.g., a low pass filter (LFP)) that extracts an envelope from the modulated RF communication signal. The amplitude (DC component) of the envelope varies depending on the distance separation between the antenna coils of the external and internal devices. Accordingly, the envelope extracted from the modulated RF signal must be properly centered prior to passing through the slicer in order to ensure that the reference voltage will slice the envelope symmetrically. Specifically, the envelope (Vin) extracted by the demodulator is received as input to a data slicer 185, as shown in FIG. 1a, that (i) centers the envelope using a capacitor 190 around a reference voltage (e.g., an average DC voltage level (Vcc/2), wherein Vcc is the power supply voltage of a processor), (ii) slices the envelope by the reference voltage (e.g., Vcc/2) using a comparator 195 to recover the digital data signal, and (iii) reshapes the digital data signal prior to being transmitted to a processor.
FIG. 1b shows, for an ideal data bit stream of alternating “1”s and “0”s, three exemplary waveforms representative of different stages in a conventional RF wireless communication system including an external device that employs a binary encoding scheme in wireless communication with an internal device. Waveform #1 represents an envelope extracted by the demodulator from a conventional binary encoded RF modulated signal for the exemplary ideal data bit stream. Thereafter, the extracted envelope is centered (as represented by waveform #2) about the reference voltage (e.g., Vcc/2) after passing through the capacitor 190. The centered envelope is symmetrically sliced and the digital signal output (as represented by waveform #3) is unaffected by the distance separation between the coils of the external and internal devices. Under these ideal conditions (i.e., a data stream comprising alternating bits) the envelope is properly centered and symmetrically sliced. Thus, the use of a conventional binary encoding scheme under these ideal conditions does not have any negative effect on the recovery of the original data stream (as represented by waveform #3).
However, a typical data stream rarely comprises exclusively alternating bits more often including strings of varying lengths of the same successive bits. When using a conventional binary encoding scheme a relatively long string of the same successive bit (e.g., “1”s or “0”s) behaves like a DC voltage that is blocked by the capacitor 190. As a result of the DC blocking the envelope will not be properly centered and thus not slice symmetrically whereby some data bits may be missed by the slicer during recovery from the RF modulated signal. FIG. 1c is an alternative scenario of a more practical data bit stream “010101000000001011111101”. In the exemplary data stream, the period of eight successive “0”s behaves as a constant DC voltage that is blocked by the capacitor 190 causing the output of the capacitor 190 (represented by waveform #2) to approach and eventually equal Vcc/2. When the voltage of the envelope output from the capacitor 190 equals Vcc/2 (as during the 13th and 14th bits) then both inputs to the comparator 195 are the same. Under these conditions the comparator output is a noise signal that toggles undesirably based on the noise levels associated with each input to the comparator. Accordingly, the exemplary reshaped and sliced digital data signal output (as represented by waveform #3) is unable to recover the original data stream during the 13th and 14th bits. This example illustrates that a string of the same successive bits may impact the recovery of one or more of those bits.
In addition, the string of successive bits may result in the drift of the baseline voltage which may effect the recovery of subsequent bits in the data stream. As clearly represented during the alternating first six bits of the data stream in which the envelope is properly centered about the baseline voltage Vcc/2, a transition in bits (e.g., from “0” to “1”, or from “1” to “0”) results in a substantially constant voltage increase/decrease after the capacitor of approximately Vcc/2. As mentioned above, after the string of 8 successive “0” bits, the baseline voltage is approximately Vcc/2. Thereafter, during the transition from “0” to “1” between the 14th and 15th bits the voltage increases from Vcc/2+Vcc/2. Next the data stream transitions between the 15th and 16th bits from a “1” to a “0” whereby the voltage decreases by Vcc/2 and returns to the baseline voltage of Vcc/2=(Vcc/2+Vcc/2)−Vcc/2, which once again due to the fact that both inputs to the comparator are the same generates noise during the 16th bit. This generation of noise during the 16th bit is therefore a result of drift of the baseline voltage which affects subsequent bits until a properly centered baseline voltage is realized.
A similar effect to that of the string of successive “0”s is encountered during the string of six successive “1”s which also behaves as a constant DC voltage that is blocked by the capacitor 190. The envelope output from the capacitor (represented by waveform #2) once again approaches Vcc/2. However, in the case of the string of six successive “1”s the 23rd bit toggles to “0” before the centered envelope (as represented by waveform #2) reaches Vcc/2.
It is therefore desirable to develop an improved passive telemetry system that overcomes the aforementioned problems by inducing a substantially constant power in the implantable medical device regardless of the bit stream being transmitted while facilitating recovery of the original data signal by ensuring a zero DC offset.